1. Field of the Invention
This invention relates to fiber telecommunication systems and, more particularly, to techniques for optimally selecting appropriate paths and setting-up connections in wavelength division multiplexed ring networks by assigning the minimum number of wavelengths that preclude color clash violations and implement shortest path routing.
2. Description of the Background Art
In today's integrated networks, it is essential that a single communications medium be able to handle traffic with different characteristics, operating in the range of a few Mb/sec to a few Gb/sec. This will enable the networks to handle such applications as large volume data or image transfers (e.g., supercomputer interconnections, supercomputer visualization, and high resolution uncompressed medical images) that have very large bandwidth requirements, as well as such applications as voice or video which require much smaller bandwidth.
The enormous potential of optical fiber to satisfy the demand for these networks has been well established over the last few decades. Optical fiber is highly reliable (Bit Error Rate (BER) in commercially deployed systems less than 10.sup.-11), it can accommodate longer repeater spacing, and it has unlimited growth potential. Single mode fiber offers a transmission medium with Tb/sec bandwidth (enough capacity to deliver a channel of 100 Mb/sec to hundreds of thousands of users), combined with high speed transmissions experiencing low loss and low bit error rates. Traditional network architectures, however, which use electrical switches and the optical fiber as a simple substitute for copper wire or other communications media, are limited by an electronic speed bottleneck and cannot be used in current networks with a growing demand for Gb/sec applications. As the next step in the evolution of the existing transport networks, Wavelength Division Multiplexed (WDM) optical networks can be deployed to provide concurrency by multiplexing a number of wavelengths for simultaneous transmission within the same medium. This approach then provides each user with a manageable portion of the enormous aggregate bandwidth.
Rapid advances in optical fiber communications technology and devices (such as filters, multiplexers/demultiplexers, fiber couplers, optical switches, multiwavelength amplifiers, Wavelength Add/Drop Multiplexers (WADM's) and Wavelength Selective Cross-connects (WSXC's)), in terms of performance, reliability and cost over the last few years, enable the future deployment of optically routed WDM networks which can be used to create high capacity nationwide broadband networks. In these networks, optical signals can flow end-to-end between users, without being converted to electrical signals at the network switches. They can offer large bandwidth, simple cross-connecting of high bit-rate streams, signal format and bit rate independent clear channels, equipment and operational savings, as well as maximum flexibility. WDM components and (mostly point-to-point) networks are currently been manufactured by a number of companies.
The WDM optical networks are envisioned to be used as transport networks for a number of technologies such as Synchronous Optical Network (SONET), Synchronous Digital Hierarchy (SDH), and Asynchronous Transfer Mode (ATM) as well as to provide end-to-end clear channels between users. Various applications utilizing these technologies or the optical network directly (clear channels) can then be provided.
An illustrative example of a WDM network is a self-healing WDM ring. Such a ring is a leading candidate architecture for high capacity local exchange carrier networks because of the survivability capabilities that the ring provides and the fact that its capacity can be shared by all the network nodes connected to the ring.
The ring network of FIG. 1 depicts an example of a self-healing ring architecture, namely, a so-called 4-fiber WDM shared protection ring 100. Two of the fibers are used as "working" fibers (carrying the network traffic) and the other two are used as "protection" fibers (dark--used only during a failure in order to restore the service). Since there are two working fibers available and the connections are always routed via shortest paths, every working fiber carries only half of the optical signal traffic on the ring. For instance, given a clockwise working fiber ring 101, a counterclockwise working fiber ring 102, along with corresponding protection fiber rings 103 and 104, the optical signal traffic from network node 111 to network node 112 can be carried in the clockwise working ring 101 and, conversely, the optical signal traffic from network node 112 to network node 111 can be carried by counterclockwise ring 102. To recover from fiber cut failures or network node failures, each operational node provides a loop-back protection switching function so that traffic on a working ring can be diverted to a protection ring to bypass the failure.
The subject matter of the present invention deals with a general ring architecture which is composed of a clockwise working ring and a counterclockwise working ring; a self-healing ring is but one example of the general principles in accordance with the present invention.
In assigning wavelengths to carry the optical signals, it is necessary to ensure that optical signals simultaneously sharing a single fiber have different wavelengths; otherwise, a so-called Color Clash (CC) occurs or, expressed equivalently, there would be in violation of the CC constraint.
In addition, it is also important practically to deploy a minimum number of wavelengths in implementing the ring network. Each additional wavelength typically requires electro-optical equipment (e.g., an optical network card including a laser) to be added in each node, thereby increasing not only the cost of connection set-up in the network but the costs of provisioning and maintenance of the network as well.
In the past, ad hoc procedures have been devised in an attempt to assign wavelengths to the required connections in the network so as to avoid Color Clash violations. As suggested, these procedures are not systematic nor scalable. These methods are not systematic because the techniques rely upon trial-and-error attempts to produce a viable wavelength assignment for a given interconnection configuration, for a given number of nodes in the ring. These methods are not scalable in the sense that if it is desired to add another node to the network with minimal disruption to the interconnections of the already-present nodes, there is no known prior art technique for adding this node in an orderly or systematic way while ensuring use of the minimum number of wavelengths, shortest path routing, and avoidance of a Color Clash. In contrast, in accordance with the present invention, the scaling technique for adding two nodes results in no disruptions, whereas the scaling technique for adding one node results in disturbing a very small number of existing connections.
A reference representative of the technological field of the present invention which discusses the ad hoc technique is the article entitled "Multiwavelength Survivable Ring Network Architectures", authored by A. F. Elrefaie and published in the Proceedings of the International Conference in Communications (ICC), 1993. Whereas this reference does address the issue of determining the minimum number of wavelengths to avoid violation of the CC constraint (for the case of odd nodes on the ring only), an ad hoc technique for assigning wavelengths to all connections on the ring is presented, which is only satisfactory for situations in which the number of nodes on the ring is an odd number. This reference does not address the case when the number of nodes on the ring is an even number. Moreover, the ad hoc method does not address a procedure for the systematic determination of wavelength assignment, nor a systematic procedure for assigning wavelengths when the ring network scales.
Thus, a need exists in the art for a systematic procedure to implement a wavelength assignment in a ring network with a minimum number of wavelengths, for either an even or odd number of nodes, in accordance to shortest path routing and without a Color Clash. Furthermore, a need exists in the art for a systematic method to assign wavelengths to the network connections whenever it is desirable/necessary to add nodes to the network.